In both of Liquid Crystal Displays (LCD) and Organic Light-Emitting Diode (OLED) displays, there is an active matrix formed by a plurality of scan lines and a plurality of data lines intersecting with each other. Taking an Active Matrix OLED (AMOLED) display as an example, there is an active matrix formed by respective scan lines and respective data lines intersecting with each other, as shown in FIG. 1, and the operating principle thereof is: turning on switching transistors on each of the respective scan lines in sequence by scanning the respective scan lines progressively, and then transmitting voltages on respective data lines to respective pixel driving transistors respectively to convert the voltages into currents for driving respective OLEDs.
A gate driving circuit (that is, a driving circuit for the scan line) is usually implemented by shift registers. Shift registers can be divided into dynamic shift registers and static shift registers. The dynamic shift register generally has a relative simple structure and contains a small amount of Thin Film Transistors (TFT), but has disadvantages of large power consumption and limited bandwidth of operation frequency. The static shift register contains a large amount of TFTs, but has advantages of low power consumption and large bandwidth of operation frequency.
As the size of a display panel increases, a gate driving circuit is often manufactured directly on the display panel, so as to reduce interconnections between the gate driving circuit and external driving circuits and thus reduce the size and cost of the display panel. A gate driving circuit designed on the basis of the display panel is generally implemented by the dynamic shift register which often adopts a dynamic circuit composed of N-type or P-type TFTs, since such designed gate driving circuit does not require a high speed but requires a compact structure and a small area.
There are various kinds of processes for manufacturing a display plane at present, for example, amorphous silicon (a—Si) TFT, Low Temperature Polycrystal Silicon (LIPS) TFT, Oxide TFT, and so on. LIPS TFT has the advantages of rapid speed and great stability, but has the disadvantages of low uniformity and high cost, and thus is not suitable for the preparation of a large scale panel. The process of a—Si TFT is relatively mature and has low cost, but the a—si TFT has the disadvantages of low mobility and low stability. Oxide TFT has the advantages of high mobility, great uniformity and low cost, but has the disadvantage of low stability. As shown in FIG. 2(a) and FIG. 2(b), during an aging test for Oxide TFT, a threshold voltage ΔVth increases gradually as an applied gate voltage Vg and the aging time increase. With the increasing of the threshold voltage of TFT, turning-on current and speed of TFT decrease. If the threshold voltage finally increases to a voltage higher than a maximum voltage applied on a gate of TFT, the TFT can not be normally turned on, causing malfunction or failure of the gate driving circuit.
A conventional shift register usually comprises a plurality of shift register sub-units connected in cascade, and each of the plurality of shift register sub-units corresponds to a scan line. As shown in FIG. 3(a) and FIG. 3(b), a conventional shift register sub-unit comprises TFTs T1 and T2 (both being N-type TFTs), wherein TFT T1 is used for transmitting a high level from a clock signal CLK to an output terminal OUT, and TFT T2 is used for transmitting a low level from a low level signal VGL to the output terminal OUT. It can be seen that, when a progressive scan is adopted, during the scan time of a frame of image (for example, if a screen refresh frequency for the display panel is 60 Hz, the scan time for a frame of image is 17 ms), each shift register sub-unit corresponding to a scan line outputs the high level from the output terminal OUT only in the scan time for the scan line, that is, a point QB is at the high level to turn on the TFT T2 at most of the time. Therefore, gates of TFTs T2 in a whole shift register circuit are applied with the high level for a long time, and thus aging speeds thereof are faster and the threshold voltages thereof increase gradually. TFT T2 can not be normally turned on if the threshold voltage thereof finally increases to an extent that the high level at the point QB is not sufficient to turn on TFT T2, causing TFT T2 failing to transmit the low level form the low level signal VGL to the output terminal OUT. Meanwhile, the output terminal of TFT is floating when TFT can not be normally turned on. At this time, if the shift register suffers from interference of external signals, the respective shift register sub-units may output the high level by mistake, causing display malfunction and low reliability of the display panel.